Correct use of virtual and OVERRIDE in HTMLTrackElement

canLoadUrl was virtual for no reason, and the TextTrackClient
implementation was missing OVERRIDE.

BUG=313287

Review URL: https://codereview.chromium.org/52443006

git-svn-id: svn://svn.chromium.org/blink/trunk@161047 bbb929c8-8fbe-4397-9dbb-9b2b20218538